Comment 3 for bug 913508

From users point of view it would be best to see a warning dialog with the option to proceed or to cancel (the most advanced version would also offer a list of associated items).

For programmers this would mean either removing the theme from all associated items or using the next level default theme as fall-back in case the the theme does not exist. I'd prefer the later solution, as it also works if users try to copy the songs.sqlite without taking the themes into account.